Advances in low-power biomedical and industrial sensor designs have made energy harvesting an attractive alternative to batteries for powering implanted or otherwise hard-to-reach sensors. Such constraints on the location of these sensors also precludes the use of solar or vibration energy harvesters as a source of energy. Thermal energy harvesting is a suitable alternative due to the presence of reasonable thermal gradients in industrial settings, or between the human body and the environment, for example. Thermoelectric generators (TEGs) can be employed for thermal energy harvesting and are capable of powering downstream electronic circuits from the TEG.
Bulk-mode TEGs typically produce 20-30 mV for every one Kelvin of temperature difference across them, with an output impedance as low as two ohms. Harvesting energy using these devices usually implies working off an output voltage as low as 50 mV in the worst case, where the temperature difference across the device is about two Kelvin. Boost converters are typically used to convert these voltages sufficiently to where CMOS circuits can subsequently be powered, for example. Starting up these converters proves to be a significant challenge since most switches in the converters have threshold voltages far exceeding the output of the TEG. In one example, this problem has been addressed by incorporating a battery to operate the switches during startup. Alternatively, a motion-activated mechanical switch can be employed. These approaches can significantly increase the cost and complexity of circuit integration and implementation, however.
In addition to the low output voltage magnitude produced by TEG's, the polarity of the induced voltage output depends on the direction of heat flow, which can vary across different applications or within the same application. Two separate transformer-based oscillators can be used in some applications for both low-voltage startup and steady-state operation—one each for each polarity. A transformer-based oscillator for startup and a transformer-based boost converter can be used in other applications, leading to higher efficiency but only with the capability to support a single polarity. Using multiple transformers also can increase the cost and complexity of a given circuit implementation.